The present invention relates to a device for holding and supplying with cooling liquid an assembly for a nuclear reactor.
More specifically, the present invention relates to a special arrangement which on the one hand ensures the maintaining in place of the assemblies forming the core of a fast neutron reactor and which on the other hand make it possible to adapt the cooling liquid flow rate passing through each assembly in order to extract the thermal calories given off by the latter.
It is known that in a nuclear reactor of this type, the core is conventionally supported by a rigid cross-member bearing against the base or ring member of a main vessel which surrounds the core and contains an appropriate volume of cooling liquid, generally sodium. The vessel may also contain, in the so-called integrated solution, heat exchangers and pumps which enable the sodium to be circulated through the core where it removes the calories given off by the fission reaction of the fuel material before restoring said calories to a secondary fluid in the heat exchangers, after which it is taken up by circulating pumps transferring it beneath the cross-member with a view to a further passage into the core. The assemblies constituted by the core are arranged vertically and each assembly comprises an elongated box open at its upper end and containing a group of sheathed fuel needles and provided at its lower end with a positioning leg. In conventional solutions, this leg serves for the fixing of the assembly on the cross-member and for regulating the flow rate of the sodium passing through the assembly.
It is also known that the core of a nuclear reactor has a central zone where the nuclear fuel within the needles of the assemblies is constituted by fissile material, said zone being completely surrounded laterally and from above and below by breeding blankets. Thus, each fuel assembly in the central zone has, in each needle, not only fissile material, but also fertile material. Moreover, the lateral blanket is constituted by assemblies similar to those of the central zone, but in which the groups of needles only contain fertile material. Finally, the latter blanket is itself surrounded by a lateral neutron protection having a plurality of concentric assembly rings which, unlike fissile or fertile fuel assemblies contains no nuclear material. These protective assemblies arranged on the core periphery have the function of preventing the activation of the secondary fluid traversing the exchangers and to minimise damage to the vessel containing the circulating liquid, sodium.
Moreover, other types of assemblies provided in the nuclear reactor core and arranged across the latter are provided and are in particular inserted in recesses made in the fuel assemblies, whereby by means of their elongated casings said assemblies constitute guidance sleeves for the regulating, control or security members necessary for the operation of the reactor. Moreover, the enrichment of the fissile material of the fuel assemblies varies according to their position in the reactor core.
It can be gathered from what has been stated hereinbefore that different types of assemblies are used in the nuclear reactor and at the regulation of the sodium flow traversing these assemblies must be adapted to their nature and their position within the reactor core.
French Pat. No. 75 20819, filed on July 2, 1975 by the present applicant relates to an assembly for a fast neutron reactor and more particularly the means for regulating the sodium flow through assembly base. In the attached FIG. 1 is shown the assembly described in said patent specification.
In FIG. 1, reference numeral 1 designates an assembly, more particularly suitable for installation in the core of a fast neutron nuclear reactor cooled by a liquid metal. This assembly has at its lower end a positioning leg, formed by a hollow cylindrical member 2, the latter being joined by a spherical bearing surface 3 to an open casing 4 arranged in the extension of the said leg. The upper end of casing 4 is terminated by a head 5, whose constructional details are unimportant with respect to the present invention. In its median part, member 2 has a zone 6 in which are provided oblong openings 7, regularly distributed about its axis, permitting the liquid cooling metal to circulate in the leg by entering the latter via these openings before flowing from bottom to top, firstly in member 2 and then in casing 4, before leaving the latter by its upper open head 5. At its lower end, member 2 is sealed by a plug 8, fixed to the leg by any appropriate means. According to a conventional arrangement, member 2 is maintained in its vertical position by engagement in a hollow shore 9, belonging to a supporting cross-member for the reactor core, diagrammatically indicated in the drawing by reference numeral 10. This shore has holes 11 located in its lateral surface to the right of opening 7 in member 2 in order to permit the liquid cooling metal delivered under pressure to cross-member 10 to enter the assembly and flow from bottom to top thereof. Member 2 rests on the upper part of shore 9 by its bearing surface 3, plug 8 engaging in a recess 8' provided on the base of the shore in the vicinity of the lower part of cross-member 10.
It is also pointed out that within leg 2 and above the oblong opening 7, the leg has on the inside diaphragms permitting the regulation of the sodium flow rate. Moreover, the inner plug 8 makes it possible to obviate errors in the fitting of the assembly due to its shape which cooperates with the shape of the recess 8'.
It can be gathered from the preceding description of the prior art that the sodium flow is regulated by arrangements forming an integral part of the actual assembly. Moreover, to ensure adequate operating conditions, the leg must have a considerable height. As a result, the weight of the fuel assembly and in particular the part of this weight due to the leg is high. However, these assemblies have to be manipulated during the loading and unloading operations of the nuclear reactor.